1. Field of the Invention
The present invention relates to a micro relay, and more particularly, to a micro relay that is activated by electrostatic attractive force and manufactured using semiconductor manufacturing techniques such as film forming, exposure, and etching.
2. Description of the Related Art
An ordinary relay is switched by current flowing in a winding provided therein. The current generates electromagnetic force that activates a contact point formed on a leaf spring. It is difficult to make the ordinary relay small and less power-consuming, however, due to such structure of the conventional relay. To solve this problem, a micro relay has been developed. The micro relay is manufactured using manufacturing processes of a semiconductor apparatus, such as film forming, exposure, and etching. The micro relay is activated by electrostatic attractive force, electromagnetic force, piezoelectric distortion, thermal expansion, and so forth. This micro relay is expected to break through the conventional limit in size and power consumption.
The micro relay is suitable for switching signal lines in which only weak current flows. One of the best applications of the micro relay is to switch high frequency signals. The micro relay is required to have a good isolation property. The isolation property indicates the amount of signals that leak between opening contacts. The smaller the amounts of signals that leak, the better isolation property the micro relay has.
An effective way to improve the isolation property is to reduce the areas of the opening contacts facing each other and to increase the distance between the opening contacts facing each other so as to reduce the electrostatic capacity connection between the opening contacts facing each other. In the case of the micro relay, the areas of the opening contacts facing each other are easily reduced. However, increasing the distance between the opening contacts facing each other is not easy since voltage that is practically applicable to the micro relay is limited to about 10 V, and the resulting activating force generated by the electrostatic attractive force is weak.
FIGS. 1 and 2 show a micro relay 10 that is disclosed in Japanese Laid-open Patent Application No. 2001-52587. The micro relay 10 is structured by laminating a fixed substrate 20, a movable substrate 30, and a cap member 40. Signal wirings 11, 12 and stationary contacts 13, 14 are formed on the top face of the fixed substrate 20. The fixed substrate 20 itself forms a stationary contact. The movable substrate 30 has a movable contact 31 on its bottom face and an upper contact unit 32 and a movable electrode 33 on its top face. The movable contact 31 and the movable electrode 33 are electrically connected to each other. The cap member 40 has a conductive layer 41 on the bottom face. When the micro relay 10 is mounted on a printed board, the conductive layer 41 is grounded.
When the micro relay 10 is activated by the applying of voltage, the electrostatic attractive force generated between the fixed substrate 20 and the movable electrode 33 bends the movable substrate 30 downward, and causes the movable contact 31 to contact the stationary contacts 13, 14. Accordingly, the signal wirings 11 and 12 are electrically connected by the movable contact 31.
When the applying of voltage to micro relay 10 is discontinued, the movable substrate 30 restores itself, and the movable contact 31 separates from the stationary contacts 13, 14. Then, the upper contact unit 32 contacts the conductive layer 41, and the movable contact 31 is grounded. Because the movable contact 31 is grounded, the electrostatic capacity between the movable contact 31 and the stationary contacts 13, 14 is eliminated. Though the distance between the movable contact 31 and the stationary contacts 13, 14 is short, the isolation property of the micro relay is good.
However, because the upper contact unit 32 contacts the conductive layer 41, the upper contact unit 32 may stick on the conductive layer 41. The electrostatic attractive force generated between the fixed substrate 20 and the movable electrode 33 by the voltage applied to the micro relay is not strong.
In situations where the upper contact unit 32 is stuck to the conductive layer 41 even in the least, the micro relay 10 is not activated even if the voltage is applied.